At least three classes of fatty acyl-coenzyme A (CoA) desaturases (delta-5, delta-6 and delta-9 desaturases) are responsible for the formation of double bonds in mono- and polyunsaturated fatty acyl-CoAs derived from either dietary sources or de novo synthesis in mammals. The delta-9 specific stearoyl-CoA desaturases (SCDs) catalyze the rate-limiting formation of the cis-double bond at the C9-C10 position in monounsaturated fatty acyl-CoAs. The preferred substrates are stearoyl-CoA and palmitoyl-CoA, with the resulting oleoyl and palmitoleoyl-CoA as the main components in the biosynthesis of phospholipids, triglycerides, cholesterol esters and wax esters (Dobrzyn and Natami, Obesity Reviews, 6: 169-174 (2005)).
The rat liver microsomal SCD protein was first isolated and characterized in 1974 (Strittmatter et al., PNAS, 71: 4565-4569 (1974)). A number of mammalian SCD genes have since been cloned and studied from various species. For example, two genes have been identified from rat (SCD1 and SCD2, Thiede et al., J. Biol. Chem., 261, 13230-13235 (1986)), Mihara, K., J. Biochem. (Tokyo), 108: 1022-1029 (1990)); four genes from mouse (SCD1, SCD2, SCD3 and SCD4) (Miyazaki et al., J. Biol. Chem., 278: 33904-33911 (2003)); and two genes from human (SCD1 and ACOD4 (SCD2)), (Zhang, et al., Biochem. J., 340: 255-264 (1991); Beiraghi, et al., Gene, 309: 11-21 (2003); Zhang et al., Biochem. J., 388: 135-142 (2005)). The involvement of SCDs in fatty acid metabolism has been known in rats and mice since the 1970's (Oshino, N., Arch. Biochem. Biophys., 149: 378-387 (1972)). This has been further supported by the biological studies of a) Asebia mice that carry the natural mutation in the SCD1 gene (Zheng et al., Nature Genetics, 23: 268-270 (1999)), b) SCD1-null mice from targeted gene deletion (Ntambi, et al., PNAS, 99: 11482-11486 (2002), and c) the suppression of SCD1 expression during leptin-induced weight loss (Cohen et al., Science, 297: 240-243 (2002)). The potential benefits of pharmacological inhibition of SCD activity has been demonstrated with anti-sense oligonucleotide inhibitors (ASO) in mice (Jiang, et al., J. Clin. Invest., 115: 1030-1038 (2005)). ASO inhibition of SCD activity reduced fatty acid synthesis and increased fatty acid oxidation in primary mouse hepatocytes. Treatment of mice with SCD-ASOs resulted in the prevention of diet-induced obesity, reduced body adiposity, hepatomegaly, steatosis, postprandial plasma insulin and glucose levels, reduced de novo fatty acid synthesis, decreased the expression of lipogenic genes, and increased the expression of genes promoting energy expenditure in liver and adipose tissues. Thus, SCD inhibition represents a novel therapeutic strategy in the treatment of obesity and related metabolic disorders.
There is compelling evidence to support that elevated SCD activity in humans is directly implicated in several common disease processes. For example, there is an elevated hepatic lipogenesis to triglyceride secretion in non-alcoholic fatty liver disease patients (Diraison, et al., Diabetes Metabolism, 29: 478-485 (2003)); Donnelly, et al., J. Clin. Invest., 115: 1343-1351 (2005)). Elevated SCD activity in adipose tissue is closely coupled to the development of insulin resistance (Sjogren, et al., Diabetologia, 51(2): 328-35 (2007)). The postprandial de novo lipogenesis is significantly elevated in obese subjects (Marques-Lopes, et al., American Journal of Clinical Nutrition, 73: 252-261 (2001)). Knockout of the SCD gene ameliorates Metabolic Syndrome by reducing plasma triglycerides, reducing weight gain, increasing insulin sensitivity, and reduces hepatic lipid accumulation (MacDonald, et al., Journal of Lipid Research, 49(1): 217-29 (2007)). There is a significant correlation between a high SCD activity and an increased cardiovascular risk profile including elevated plasma triglycerides, a high body mass index and reduced plasma HDL (Attie, et al., J. Lipid Res., 43: 1899-1907 (2002)). SCD activity plays a key role in controlling the proliferation and survival of human transformed cells (Scaglia and Igal, J. Biol. Chem., (2005)). RNA interference of SCD-1 reduces human tumor cell survival (Morgan-Lappe, et al., Cancer Research, 67(9): 4390-4398 (2007)).
Other than the above mentioned anti-sense oligonucleotides, inhibitors of SCD activity include non-selective thia-fatty acid substrate analogs [B. Behrouzian and P. H. Buist, Prostaglandins, Leukotrienes, and Essential Fatty Acids, 68: 107-112 (2003)], cyclopropenoid fatty acids (Raju and Reiser, J. Biol. Chem., 242: 379-384 (1967)), certain conjugated long-chain fatty acid isomers (Park, et al., Biochim. Biophys. Acta, 1486: 285-292 (2000)), and a series of heterocyclic derivatives disclosed in published international patent application publications WO 2005/011653, WO 2005/011654, WO 2005/011655, WO 2005/011656, WO 2005/011657, WO 2006/014168, WO 2006/034279, WO 2006/034312, WO 2006/034315, WO 2006/034338, WO 2006/034341, WO 2006/034440, WO 2006/034441, WO 2006/034446, WO 2006/086445; WO 2006/086447; WO 2006/101521; WO 2006/125178; WO 2006/125179; WO 2006/125180; WO 2006/125181; WO 2006/125194; WO 2007/044085; WO 2007/046867; WO 2007/046868; WO 2007/050124; WO 2007/130075; WO 2007/136746; and WO 2008/074835, all assigned to Xenon Pharmaceuticals, Inc. SCD inhibitors are also disclosed in the following published international patent application publications: WO 2008/074835; WO 2008/074824; WO 2008/036715; WO 2008/044767; WO 2008/029266; WO 2008/062276; and WO 2008/127349.
A number of international patent applications assigned to Merck Frosst Canada Ltd. that disclose SCD inhibitors useful for the treatment of obesity and Type 2 diabetes have also published: WO 2006/130986 (14 Dec. 2006); WO 2007/009236 (25 Jan. 2007); WO 2007/056846 (24 May 2007); WO 2007/071023 (28 Jun. 2007); WO 2007/134457 (29 Nov. 2007); WO 2007/143823 (21 Dec. 2007); WO 2007/143824 (21 Dec. 2007); WO 2008/017161 (14 Feb. 2008); WO 2008/046226 (24 Apr. 2008); WO 2008/064474 (5 Jun. 2008); and US 2008/0182838 (31 Jul. 2008).
WO 2008/003753 (assigned to Novartis) discloses a series of pyrazolo[1,5-a]pyrimidine analogs as SCD inhibitors; WO 2007/143697 and WO 2008/024390 (assigned to Novartis and Xenon Pharmaceuticals) disclose heterocyclic derivatives as SCD inhibitors; and WO 2008/096746 (assigned to Takeda Pharmaceutical) and WO 2008/056687 (assigned to Daiichi) disclose spiro compounds as SCD inhibitors.
Small molecule SCD inhibitors have also been described by (a) G. Liu, et al., “Discovery of Potent, Selective, Orally Bioavailable SCD1 Inhibitors,” in J. Med. Chem., 50: 3086-3100 (2007); (b) H. Zhao, et al., “Discovery of 1-(4-phenoxypiperidin-1-yl)-2-arylaminoethanone SCD 1 inhibitors,” Bioorg. Med. Chem. Lett., 17: 3388-3391 (2007); and (c) Z. Xin, et al., “Discovery of piperidine-aryl urea-based stearoyl-CoA desaturase 1 inhibitors,” Bioorg. Med. Chem. Lett., 18: 4298-4302 (2008).
The present invention is concerned with novel heteroaromatic compounds as inhibitors of stearoyl-CoA delta-9 desaturase which are useful in the treatment and/or prevention of various conditions and diseases mediated by SCD activity including those related, but not limited, to elevated lipid levels, as exemplified in non-alcoholic fatty liver disease, cardiovascular disease, obesity, diabetes, metabolic syndrome, and insulin resistance.
The role of stearoyl-coenzyme A desaturase in lipid metabolism has been described by M. Miyazaki and J. M. Ntambi, Prostaglandins, Leukotrienes, and Essential Fatty Acids, 68: 113-121 (2003). The therapeutic potential of the pharmacological manipulation of SCD activity has been described by A. Dobrzyn and J. M. Ntambi, in “Stearoyl-CoA desaturase as a new drug target for obesity treatment,” Obesity Reviews, 6: 169-174 (2005).